The present invention relates to a method for activating a focusing servo that automatically makes adjustment so that the focal point of a light irradiated onto a disc tracks the recording surface of the disc in order to read/write information from/onto the disc as a disc-shaped recording medium, and disc apparatus comprising at least one of a feature to regenerate information from a disc and a feature to record information onto a disc.
A focusing error signal FE indicating the amount of dislocation of the focal point of a light irradiated onto a disc from the recording surface of the disc typically varies depending on the relation between the focal point of a light irradiated onto the disc and the disc, for example as shown in FIG. 6. That is, in the state where the focal point of a light irradiated onto a disc is on the recording surface (hereinafter referred to as “in-focus state”), the focusing error signal FE obtains a value of 0. When the focal point is in close proximity of the in-focus state, the absolute value in the positive polarity of the signal increases as the focal point moves away from the recording surface of a disc. The absolute value in the negative polarity of the signal increases as the focal point approaches the recording surface of a disc. When the focal point is not in close proximity of the in-focus state, the absolute value of the signal becomes smaller as the focal point approaches the recording surface of a disc. Finally, the spot size becomes larger than the light-receptive surface thus providing a value of 0.
In order to let a focus servo so that the in-focus state is maintained, the focusing servo must be activated in close proximity of the in-focus state. In order to realize this, the basic concept is that the in-focus state is detected at the zero-crossing point of the focusing error signal FE (the point where the focusing error signal FE passes through 0) in order to activate the focusing servo.
However, simply activating the focusing servo at the zero-crossing point of the focusing error signal FE results in erroneous detection of the in-focus state at the zero-crossing point P′ of a noise N (a point not in close proximity of the in-focus state) thus activating the focusing servo in case focusing error signal FE contains the noise N. This causes the focusing servo to maintain a state other than the in-focus state. In order to solve this problem, a focusing servo used to be activated at a zero-crossing point P of the focusing error signal FE immediately after the focusing error signal FE has exceeded a fixed threshold th.
From the viewpoint of preventing erroneous detection of an in-focus state caused by a noise factor, the threshold is desirably as large as possible. On the other hand, the amplitude of a focusing error signal is varied depending on the individual difference of a photo-detector. In case the threshold is too large, the focusing error signal does not exceed the threshold so that it is not possible to activate a focusing servo.
Thus, the threshold must be set considering the variation in the amplitude of the focusing error signal so that the focusing servo may be activated without being affected by the variation in the amplitude of the focusing error signal. The greater the variation in the amplitude of the focusing error signal, the smaller the threshold must be, in order to correctly detect the in-focus state. Setting too small a threshold causes erroneous recognition of a noise as the in-focus state.
While the Unexamined Japanese Patent Application Publication No. Hei3-156731 discloses an invention whereby only a focusing error signal is used to correctly and stably detect an in-focus state, the aforesaid problem arises because the threshold is fixed.